EFFECT OF DYE STRUCTURE ON AFFINITY FOR FIBERS 123 



PLANARITY AND AFFINITY 



SOsNa 



HaN 



SOsNo 



^"2 chJ) (5cH3 h2nO 



SOsNa 



HIGH AFFINITY 



NO AFFINITY 



C^H?<^^-^° 



SOsNa 



HIGH AFFINITY 

 H2N 

 Fig. 11 



in geometry. The next figure (Fig. 13) illustrates a case where a change in ar- 

 rangement of groups appears to prevent the molecule from layering properly 

 on cellulose and thus diminishes its affinity. 



Dyeing of cellulose is, therefore, a complex subject — best explained at present 

 by assuming association between polar molecules and a polar substrate with 

 indications that an exact fit rather strongly increases affinity. 



Another mechanism comes into play in dyeing fibers containing ionic groups. 

 Both hydrophobic and hydrophilic polymers with ionizable groups are dyed by 

 a reaction which results in the formation of ion pairs and the consecjuent at- 

 tachment of the dye ions at specific ionic sites. The physical chemistry has been 

 worked out in detail by Gilbert and Rideal for the absorption of acids by wool, 

 by Remington and Gladding for the absorption of dye anions by nylons, and by 

 Blaker, Katz, and co-workers for the absorption of dye anions by polyacryloni- 

 trile containing cationic sites. In each of these cases the observed isotherms are 

 of the limited site type, and the reaction is one of the ion pair formation. As a 

 matter of fact each of these polymers absorbs colored anions in much the same 

 way that an ion exchange resin absorbs colorless ions. In some cases the dyeing 

 reaction is a simple ion exchange in which the more affinitive dye anion displaces 

 a simple inorganic anion such as bisulfate. 



Considering the case of wool and nylon, which are amphoteric materials, 

 dyeing is regarded as attachment of dye anions on the protonated amines. For 

 these cases thermodynamic expressions have been derived expressing the sim- 

 ple ionic reactions. These predict a linear relationship. For the absorption of 

 metanil yellow on nylon the experimental data give a straight line whose inter- 

 cept with the abscissa is an exact measure of the concentration of amino groups, 

 the sites to which dye and proton become affixed. 



